This invention relates to polyester fibers having high boiling water shrinkage, i.e., at least 40%, yarns made therefrom, and a method of producing the high shrinkage polyester fibers.
Polyester fibers have been prepared for commercial use for more than thirty years, and are produced in large quantities. Most commercial polyester comprises poly(ethylene terephthalates).
The term "fiber" as used herein includes fibers of extreme or indefinite length (i.e., filaments) and fibers of short length (i.e., staple). The term "yarn", as used herein, means a continuous strand of fibers.
Because fibers produced from polyester have a number of outstanding characteristics: excellent dimensional stability and sturdiness, a high degree of crease resistance, good bulk elasticity, and warm handle, the fibers made from polyester have found a wide variety of applications, especially in the textile field.
Polyester fibers are normally produced having a reduced final shrinkage. However, in certain applications, it is desirable for the polyester fibers to have a high shrinkage. For instance, since polyester fibers tend to have a "crushing problem", or, in other words, when an object of sufficient weight is placed on a fabric comprising polyester fibers, the contour of the object tends to remain on the fabric after the object is removed. This problem is particularly acute for fabrics made from polyester fibers which are used for automotive upholstery. In this application, the weight of an object, such as a person, produces a profile of the object after the weight of the object has been removed. This result affects the aesthetic qualities of the product containing the polyester fibers. Therefore, there is a need in the art to provide polyester fibers which overcome or at least mitigate this problem.
In addition, it is sometimes desirable to blend polyester fibers having low shrinkage with polyester fibers having high shrinkage to produce a resulting product in which bulk is developed along with a soft handle.
Procedures have been utilized in the past to produce high shrinkage polyester fibers. Problems associated with these procedures are that, many times, strength or uniform dyeability or combinations of these properties are adversely effected in producing the high shrinkage polyester fibers.
Thus, the combined objective of polyester fibers having high shrinkage, uniform dyeability, good light stability, and good strength becomes somewhat irreconcilable in many of the processes for producing polyester fibers.
The present invention produces high shrinkage polyester fibers and yarns made therefrom which have an improved combination of properties, i.e., good strength and uniform dyeability and a method of producing the high shrinkage polyester fibers having the improved combination of properties, i.e., one which involves less sacrifice of one or more individual properties to improve the other.
It has been unexpectedly discovered that yarn comprising poly(ethylene terephthalate) fibers having the above-described combination of properties can be prepared from a partially oriented feeder yarn comprising poly(ethylene terephthalate) fibers having a birefringence ( n) of at least 0.0175 by drawing the feeder yarn at a draw ratio in the range of from about 1.98 to about 2.10 and at a temperature (20.degree.-25.degree. C.) below the glass transition temperature of poly(ethylene terephthalate). This is especially surprising, since it has been heretofore unknown that polyesters could be successufully uniformly cold drawn, i.e., drawn below the glass transition temperature.
The poly(ethylene terephthalate) filaments produced are characterized by a boiling water shrinkage of at least 40%, low crystallization, usually 15 to about 20 percent, a tenacity of 4.0 to 5.0 grams per denier, a long-period spacing (LPS) of greater than 225 .ANG.. Preferably, the filaments have an average crystal size in the range of from about 25 to about 30 .ANG. as measured in the direction of the fiber axis (105).